Mineral-Specific Controls on Pore-Scale Remaining Oil Distribution in Sandstone after Waterflooding

Summary Understanding how mineral composition controls the pore-scale distribution of remaining oil is essential for improving waterflooding efficiency and future tertiary oil recovery strategies in sandstone reservoirs. In this study, nuclear magnetic resonance (NMR) in-situ displacement experiments were first conducted on Berea sandstone and Daqing reservoir sandstone to evaluate the role of mineralogy in governing remaining oil wettability. Post-waterflooding cores were prepared into large-area planar samples using cryogenic mechanical cutting and polishing-broad ion beam polishing (Cryo-MCP-BIB), followed by cryogenic scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) imaging to achieve high-resolution mineral-fluid characterization. Remaining oil was classified into five categories—droplet, cluster, blind-end, membranous, and irregular—based on quantitative analysis of oil volume, shape factor, contact ratio, pore-scale position, and pore saturation. Droplet, blind-end, and cluster oil were mainly distributed in large pores bounded by quartz and feldspar, separated from mineral surfaces by nanometer-scale water films. Membranous and irregular oil was primarily associated with clay minerals, highlighting the strong mineralogical control on oil trapping. NMR and microcomputed tomography (micro-CT) results confirmed that Daqing reservoir sandstone is overall water-wet, with a weakly water-wet contact angle distribution but localized oil-wet behavior (>90°) driven by mineralogical heterogeneity. Cryo-MCP-BIB-SEM revealed contact angles >100° on quartz surfaces, while feldspar remained strongly water-wet (10–50°). Localized oil-wet quartz surfaces strongly influenced oil distribution, particularly in pore corners where membranous oil trails developed. Mineral-specific wettability contrasts were further evident in clay minerals. Kaolinite exhibited two oil/water interfacial configurations, embedded within oil phases and residing at oil/water boundaries, while chlorite displayed three—separation by water films, direct pinning, and partial contact with both phases. During waterflooding, chlorite-associated water films promoted snap-off and ganglia formation but also reduced adhesion, facilitating oil displacement. Overall, this work reveals direct nanoscale evidence that mineral-specific wettability heterogeneity, coupled with pore geometry, directly controls remaining oil morphology, distribution, and displacement efficiency at the pore scale.